The overall number of defects in open-source software projects is dropping, according to a new study by San Francisco-based vendor Coverity Inc.

Coverity, a maker of tools for analyzing programming code, received a contract in 2006 from the U.S. Department of Homeland Security to help boost the quality of open-source software, which is increasingly being used by government agencies for critical applications.

The vendor set up a Web site where open-source developers can submit code to be analyzed. A project is ranked on a scale of "rungs," based on how many defects have been resolved.

"Defect density" has dropped 16% over the past three years among the projects scanned through the site, and 11,200 defects have been eliminated, according to Coverity's latest report.

Four projects have been awarded the top-level Rung 3 status for resolving defects discovered in the previous stages: Samba, Tor, OpenPAM and Ruby.

Coverity's Scan site so far has analyzed more than 60 million unique lines of code from 280 projects.

The company's scanning service uses static analysis, which checks code for security or performance problems without having to run an application. "Static analysis [tools] won't tell you that your business process is working correctly ... but they will tell you that the code itself is technically solid and follows the kind of programming best practices you'd expect to see from code that has gone through a proper code review," said Forrester Research Inc. analyst Jeffrey Hammond, via e-mail.

He said the tools are most helpful for finding structural problems, such as memory leaks and buffer overflows, caused by poor programming practices, as well as more exotic conditions like errors caused by parallel execution of code in a multicore CPU environment.